EP0243937A1

EP0243937A1 - Variable-speed pumped-storage power generating system

Info

Publication number: EP0243937A1
Application number: EP87106153A
Authority: EP
Inventors: Akihiro Sakayori; Takao C/O Hitachi Ltd. Kuwabara; Akira Bando; Yasuteru Ono; Shigeaki Hayashi; Isao Yokoyama; Kenzyu Ogiwara
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1986-04-30
Filing date: 1987-04-28
Publication date: 1987-11-04
Anticipated expiration: 2007-04-28
Also published as: US4816696A; DE3770332D1; EP0243937B1; CN87103153A; KR870010677A; IN168574B; CN1006924B; KR920007070B1

Abstract

A variable-speed pumped-storage power generating system includes a variable-speed driver composed of a generator/motor (l) and a frequency converter (4) connected to an electric power system (5) and a pump/turbine (6) driven by the generator/motor. The power generating system comprises an optimum rotation speed function generator (l2) calculating, on the basis of at least an externally applied output command signal, an optimum rotation speed of the motor for driving the pump with high efficiency, a speed detector (2) generating a speed detection signal by detecting the actual rotation speed of the motor, a speed regulator (l6) receiving the output signals of the function generator and speed detector to generate an output correction signal for decreasing the speed error to zero, an adder (l0l) adding the output correction signal to the output command signal, a comparator (l02) comparing the output signal of the adder with an output power detection signal generated from a motor output detector (40) and generating an error signal therebetween, and an output regulator (8) controlling the excitation of the secondary winding of the motor through the frequency converter to decrease the output power error to zero, thereby controlling the output and rotation speed of the motor. The power generating system also comprises an optimum guide-vane opening function generator (l3) calculating an optimum guide vane opening on the basis of at least the output command signal, a comparator (l03) comparing the output signal of the second function generator with a guide-vane opening detection signal from a guide-vane opening detector (70), and a guide-vane opening regulator (9) controlling the opening of the guide vanes according to the result of comparison.

Description

BACKGROUND OF THE INVENTION

This invention relates to a variable-speed pumped-storage power station, and more particularly to a variable-speed pumped-storage power generating system which is suitable to stably continue pumping operation under command of an output command signal.
A variable-speed pumped-storage power generating system is commonly known from the disclosure of, for example, United States Patent No. 4,48l,455. The disclosed system includes a generator motor which has a primary winding connected to an a.c. power system through a main circuit including a breaker and a main transformer. The generator motor is directly coupled at its rotor to a prime mover/load and has a secondary winding connected to the a.c. power system through an excitation circuit including a frequency converter and an exciting transformer. The system disclosed in the cited patent has such a great advantage that the prime mover/load can be driven at a rotation speed independent of the frequency of the a.c. power system. In an application of the power generating system to a variable-speed pumped-storage power station, a turbine/pump is coupled directly to the rotor of the generator motor as the prime mover/load, and, during generating operation, the rotor of the generator motor (the generator) is driven by the water turbine, thereby inducing power in the primary winding of the generator motor. The induced power is supplied to an electric power system. On the other hand, during motoring operation (pumping operation), the generator motor is driven as the motor by the power supplied from the electric power system, and water is pumped up by the pump coupled directly to the rotor of the generator motor. In this case, the frequency of the electric power system is constant or, for example, 60 Hz. However, the rotation speed of the turbine/pump can be freely selected independently of the frequency of the electric power system. Thus, the rotation speed is selected to maximize the operation efficiency of the water turbine or pump.
The fact that the rotation speed N of the turbine/pump can be freely selected means that there is a slip frequency f₂ between the frequency f₁ of the electric power system and the frequency
P: the number of poles of winding) corresponding to the rotation speed N of the turbine/pump, and the following equation (l) holds:
The slip frequency f₂ in the equation (l) is the frequency of the secondary winding of the generator motor, and the rotation speed N can be set at a value which maximizes the pump efficiency or turbine efficiency while maintaining constant the frequency f₁ of the electric power system. There are two principal manipulated variables that can be regulated to satisfy the equation (l). One of them is the opening of inlet valves or guide vanes in a conduit leading to the turbine/pump, and the other is the firing angle of thyristors constituting the frequency converter. These variables are suitably controlled according to a power command signal determined by the operation mode of the variable-speed pumped-storage power station.
United States Patent No. 4,48l,455 cited above does not refer to a manner of concrete control of such a variable-speed pumped-storage power station. In this connection, a patent application, for example, JP-A-60-9099l (corresponding to United States Patent Application Serial No. 664436) discloses a control apparatus suitable for controlling such a power station although its principal intension is control of the power station during generating operation. According to the disclosure of the known patent application, an output command signal and a turbine's head signal are applied as inputs to function generators which generate an optimum speed command signal and an optimum valve opening command signal respectively. A signal representing the actual rotation speed of the water turbine corresponding to the former command signal is fed back to control the firing angle of the thyristors of the frequency converter, and a signal representing the actual opening of the turbine's guide vanes corresponding to the latter command signal is fed back to control the opening of the guide vanes of the water turbine. Further, a frequency variation or power variation in the electric power system is detected to correct the output command signal thereby consequently correcting the opening of the guide vanes. The disclosed control apparatus is intended exclusively for controlling the generating operation of the power station, and the output command signal applied as an input is used merely as an auxiliary signal for obtaining a target speed signal and a target guide-vane opening signal. That is, the output command signal is not directly compared with an actual generator output. Thus, from the viewpoint of output control (of the generator), open loop control systems are provided for the speed control and guide-vane opening control, and the generator output is determined as a result of the above controls. From the viewpoint of the generator output control, such open loop control systems, in which the rotation speed control is a primary object, and the generator output control is a secondary object, do not quickly respond but respond with a delay time. Further, although plural control systems such as the rotation speed control and the generator output control are desired sometimes to be applied to the same generator any practical counter-measure against interference therebetween has not been realized yet. In the nighttime where the load of the electric power system is light, the greater part of the load is supplied from a nuclear power station or a large-capacity thermal power station which must make base load operation, hence, which has not a frequency regulatability, and the remaining part is borne by a hydro-electric power station or other thermal power station operable with a low generation cost. That is, for the purpose of an economical use of the electric power system, the thermal power station requiring a high generation cost is shut down or operates at a light load in the nighttime. Because of such a middle load operation, shortage of the electrical output and frequency regulatability in the nighttime has resulted. Suppose, for example, a case where shortage of electrical power in the nighttime must be immediately covered. In such a case, a period of time of at least several minutes to ten-odd minutes is required to increase the output of the thermal power station operating under the light load, and such a long time required for covering the shortage of power is not useful for the purpose of emergency control. That is, the function of regulating both the quantity of supplied power and the power system frequency according to a load variation (the so-called automatic frequency control function) is not fully exhibited.
A pumped-storage power station of variable-speed type provides various merits. One of the merits is that the power station can operate at a high efficiency in each of the generating operation mode and the pumping operation mode. According to another merit, the automatic frequency control (AFC) function is exhibited even when the power station operates in the pumping mode, that is, when the generator motor operates as the motor. These are most desirable merits in use of the pumped-storage power station of the variable-speed type.
However, the known control system, which is based only on the premise that the rotation speed is controlled by controlling the output of the motor, is not satisfactory in its response speed and stability in that the peculiar AFC function cannot be fully exhibited in the pumping operation in the nighttime.

SUMMARY OF THE INVENTION

With a view to obviate the prior art inability of fully exhibiting the AFC function, it is a primary object of the present invention to provide a variable-speed pumped-storage power generating system suitable for fully exhibiting the AFC function during pumping operation and executing both the drive output control and the rotation speed control for the motor independently of each other and without contradiction.
In accordance with one aspect of the present invention, there is provided a variable-speed pumped-storage power generating system including a generator motor having a primary winding connected to an electric power system and a secondary winding connected to the electric power system through a frequency converter, and a turbine/pump directly coupled to the shaft of the generator motor, the variable speed pump or pump-generating system comprising means for producing a target power signal by adding an error signal representing the difference between an actual rotation speed and a target rotation speed of the generator motor to an output command signal commanding an output power required for the variable speed pump or pump-generating system, and controlling an internal phase difference angle (a phase difference between an internal induced voltage and a terminal voltage) through the frequency converter according to an error signal between the target power signal and a power feedback signal feeding back an actual output power of the generator-motor, the output command signal including also a command signal from an AFC device installed in a central load-dispatching office generally controlling the electric power and pump system or such control signal for the electric power system.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. l shows diagrammatically the structure of a variable-speed pumped-storage power generating system to which the present invention is applied.
Fig. 2 is a block diagram showing the strusture of a typical embodiment of the present invention.
Figs. 3 and 4 are graphs showing the characteristics of the function generators l2 and l3 shown in Fig. 2 respectively.
Figs. 5 and 6 are block diagrams of modifications showing other manners of calculating the optimum opening of the guide vanes.
Figs. 7 and 8 show how various status quantities in Fig. 2 change when the output command signal P_o is changed.
Fig. 9 shows diagrammatically the structure of another variable-speed pumped-storage power generating system to which the present invention is also applicable.
Fig. l0 shows one form of a function generator suitable for starting the pumping operation.
Fig. ll shows one form of a function generator suitable for quickly passing over the synchronous speed N₁.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. l shows diagrammatically the structure of a variable-speed pumped-storage power generating system to which the present invention is applied.
Referring to Fig. l, a generator motor l directly coupled to a turbine/pump 6 has a primary winding lP and a secondary winding lS. The primary winding lP is connected to an electric power system 5 through a main circuit l00 including a breaker ll and a main transformer 2l. The secondary winding lS is connected to the electric power system 5 through an excitation circuit 200 including a frequency converter 4, an exciting transformer 23 and a breaker l4. An output controller l0 generates a control voltage signal l30 on the basis of an output command signal P_o applied thereto and applies the signal l30 to an automatic pulse-phase shift controller 3. On the basis of the signal l30, the automatic pulse-phase shift controller 3 generates and applies a firing signal l3l to thyristors constituting the frequency converter 4. A guide vane controller 30 is provided to control the opening of guide vanes 7 of the water turbine 6. In order to detect various plant variables in the variable-speed pumped-storage power generating system, there are a detector 40 detecting the electric power P_r, another detector 2 detecting the rotation speed N_r of the turbine/pump 6, another detector 70 detecting the opening Y_r of the guide vanes 7, and another detector (not shown) detecting the gross head H_ST of the turbine/pump 6. A potential transformer PT and a current transformer CT are connected to the power detector 40.
A typical embodiment of the present invention will now be described with reference to Fig. 2. Referring to Fig. 2, the output controller l0 receives as inputs an output command signal P_o from a central load-dispatching office and/or a local setter, a gross head detection signal H_ST from the head detector (not shown), a pump rotation speed signal N_r from the pump speed detector 2, and a load detection signal P_r from the power detector 40, and applies a control voltage signal l30 to the APPSC (Automatic Pulse Phase Shift) 3 as an output. An opening detection signal Y_r from the guide-vane opening detector 70 is applied as an input to the guide-vane opening controller 30 in addition to the signals P_o and H_ST. On the basis of these input signals, the controller 30 determines the opening of the guide vanes of the turbine/pump 6. A guide vane regulator 9 is a sort of a hydraulic amplifier and normally makes an integrating operation. As far as there is a difference between the signals Y_OPT and Y_r, the guide vane regulator 9 integrates the difference between Y_OPT and Y_r negatively fed back from the vane opening detector 70 until Y_r becomes equal to Y_OPT. The generator-motor l and the turbine/pump 6 constitute a rotary mechanical system 50 together with an adder part l04 and an inertia effect part l05. It will be seen that the rotation speed N_r of the turbine/pump 6 is determined by the difference between the input and output torques of the rotary mechanical system 50.
The present invention shown in Fig. 2 is featured in that the power control system is constructed so that the actual drive power can directly follow up the output command signal P_o and is formed as part of the secondary excitation control system of the generator motor l. Further, the present invention is featured in that a signal ε for controlling the rotation speed of the generator motor l is added to the output command signal P_o as a correction signal. In Fig. 2, the output command signal P_o is applied to the output controller l0 from, for example, the central load-dispatching office and includes an AFC signal besides an ELD (economical load dispatch) signal etc. The output controller l0 responds quickly to the output command signal P_o including the AFC signal which change incessantly. Another input to the output controller l0 is the signal indicative of the gross head H_ST. This gross head H_ST simply represents the water level difference between an upper reservoir and a lower reservoir for the turbine/pump 6. On the other hand, the net pump head H is defined as the sum of the gross head H_ST and a conduit loss in the pumping system. Thus, when the condition of location of the pumping system is such that there is little variation in the water level (variation in the gross head H_ST) during operation of the turbine/pump 6, the gross head H_ST need not be detected by the H_ST detector (not shown) and can be handled as a constant value. In such a case, it is apparent that function generators l2 and l3 described below are only required to respond to the output command signal P_o and may be made simpler in structure.
The output controller l0 includes an optimum rotation speed function generator l2 calculating an optimum rotation speed N_OPT of the turbine/pump 6 at the application timing of the signals P_o and H_ST, a comparator l00 comparing the optimum rotation speed N_OPT provided by the output signal from the function generator l2 with an actual rotation speed N_r detected by the speed detector 2, and a speed regulator l6 including at least an integrating element for decreasing the speed error to zero. The circuits described above constitute a speed control system, and an output correction signal ε appears from the speed regulator l6.
The output correction signal ε is added to the output command signal P_o in an adder l0l to provide a composite output command signal (P_o + ε), and this composite signal (P_o + ε) is compared with an actual drive output P_M in a comparator l02. An output regulator 8 including at least an integrating element is connected to the comparator l02 so as to provide a negative feedback circuit to decrease the error between P_M and (P_o + ε) to zero. The circuits described above constitute an output control system. The output signal of the output regulator 8 provides the control voltage signal l30 applied to the APPSC 3 to determine an internal phase difference angle through the thyristors of the frequency converter 4 thereby controlling the drive output P_M of the motor l. This drive output P_M is detected by the power detector 40 as P_r which is used for the negative feedback control of the output.
As in the case of the output controller l0, the output command signal P_o and the gross head signal H_ST are applied to the guide-vane opening controller 30. The guide-vane opening controller 30 includes an optimum guide-vane opening function generator l3 calculating an optimum guide-vane opening Y_OPT at the application timing of the output command signal P_o and gross head signal H_ST, a comparator l03 comparing the optimum guide-vane opening Y_OPT provided by the output signal from the function generator l3 with an actual guide-vane opening Y_r detected by the guide-vane opening detector 70, and a guide vane regulator 9 including at least an integrating element for decreasing the error between Y_OPT and Y_r to zero. By the above manner of control, a mechanical input P_p required for the turbine/pump 6 is determined. When the actual drive output P_M of the motor l does not coincide with the mechanical input P_p required for the turbine/pump 6 in the rotary mechanical system 50, the difference or error (P_M - P_p) detected by a comparator l04 appears through the inertia effect part GD² l05 as a speed change. As a result, the error (P_M - P_p) is detected by the speed detector 2, and the drive output P_M of the motor l is controlled until the speed signal N_r becomes equal to its target speed signal N_OPT.
Figs. 3 and 4 show the characteristics of the function generators l2 and l3 respectively. In these graphs, the values of N_OPT and Y_OPT required to optimize the efficiency of the turbine/pump are determined as a function of H_ST and P_o as a result of a model test on the turbine/pump. It can be understood from Fig. 3 that the larger the value of P_o, the value of N_OPT is larger when H_ST is constant, and the larger the value of H_ST, the larger is the value of N_OPT when P_o is constant. Also, it can be understood from Fig. 4 that the larger the value of P_o, the larger is the value of Y_OPT when H_ST is constant, and the larger the value of H_ST, the smaller is the value of Y_OPT when P_o is constant. Therefore, when the output command signal P_o increases within a short period of time in which any appreciable variation does not occur in the gross head H_ST, such an increase in P_o can be sufficiently dealt with by increasing both of N_OPT and Y_OPT. Similarly, a decrease in P_o can be sufficiently dealt with by decreasing both of N_OPT and Y_OPT. This means that the manner of control is to be such that a change in P_o is dealt with by changing both of P_M and P_p in the same direction. That is, P_p is increased when P_M is increased. Also, an increase in H_ST when P_o is constant is dealt with by decreasing Y_OPT and increasing N_OPT.
Figs. 5 and 6 show modifications of the block diagram shown in Fig. 2. In each of modifications shown in Figs. 5 and 6, the manner of signal processing in the output controller l0 remains unchanged, and the manner of calculation of the optimum guide-vane opening Y_OPT in the guide-vane opening controller 30 deffers from that of Fig. 2. In Fig. 2, the optimum guide-vane opening Y_OPT is calculated on the basis of the gross head signal H_ST and output command signal P_o. In contrast, in Figs. 5 and 6, the optimum guide-vane opening Y_OPT is calculated on the basis of the gross head signal H_ST and the speed command signal or an actual speed. That is, in Fig. 5, the detected speed N_r is used for calculation of Y_OPT, while in Fig. 6, the output N_OPT of the function generator l2 is used for the calculation of Y_OPT. In each case, a function generator l7 having a characteristic as shown in Fig. 7 is used for the calculation of Y_OPT. It is apparent that, in this case too, changing directions of the drive output P_M and the mechanical input P_p due to a change in P_o or H_ST are similar to those described with reference to Fig. 2.
The embodiments shown in Figs. 5 and 6 differ only from that shown in Fig. 2 in the manner of calculation of the optimum guide-vane opening Y_OPT, and they are identical to the invention disclosed in Fig. 2 in the technical idea of the present invention in which a correction is applied to the generator motor drive output control system from the rotation speed control system. Further, as described already, the response of the output control system is very quick as compared to that of the guide-vane opening control system. The attain ability of the object of the present invention (which contemplates to provide a variable-speed pumped-storage power generating system suitable for full exhibition of the AFC function especially during pumping operation) will be described by reference to the typical embodiment of the present invention shown in Fig. 2.
Suppose, for example, that an oversupply of electric power occurs in the electric power system including the variable-speed pumped-storage power stations, and an increase in the power system frequency results. In such an event, the central load-dispatching office applies an output-decrementing command signal to the power stations and applies also a drive output (electrical load)-incrementing command signal P_o to the variable-speed pumped-storage power station of the present invention operating in its pumping mode. In response to this drive output-incrementing command signal P_o, the variable-speed pumped-storage power generating system shown in Figs. l and 2 operates as described below. It is supposed herein that the plant variables N_r, P_r and Y_r at various parts in Fig. 2 are set at N_r = N_OPT by the speed control system, P_M = P_r = P_o and ε = O by the output control system, and Y_r = Y_OPT by the guide vane control system, before the output command signal P_o is incremented from its constant value. There holds the relation P_M = P_p between the mechanical input P_p required for the turbine/pump 6 and the actual drive output P_M of the motor l. This is because the difference (P_M - P_p) becomes zero due to the inertia effect GD² (l05) of the combination of the motor l and the turbine/pump 6, which can be regarded as a kind of an integrating element. Therefore, when errors that may occur in the function generators l2 and l3 are ignored, Y_OPT and N_OPT are equivalent to P_o, or the relations Y_OPT = P_o or equivalent and N_OPT = P_o or equivalent hold. Thus, the relation P_o = Y_OPT or equivalent = Y or equivalent = P_p = P_M holds, and the output correction signal ε is zero.
When, under such a state, the drive output command signal P_o from the central load-dispatching office shows a sharp increase at time t₁ as shown in Fig. 7(a), the actual drive output P_M increases relatively quickly as described later, and, on the other hand, the rotation speed command signal N_OPT generated from the function generator l2 increases almost instantaneously, as will be apparent from Fig. 3 showing the characteristic of the function generator l2. On the other hand, the actual rotation-speed detection signal N_r cannot immediately increase up to the level of the signal N_OPT due to the inertia effect or the like as described later, and an error occurs therebetween. This error is integrated by the integrating function included in the speed regulator l6 to appear as the correction signal ε. This correction signal ε is added to the drive output command signal P_o in the adder l0l to provide the composite drive output command signal (P_o + ε) for the generator motor l. This command signal (P_o + ε) is compared with the actual output detection signal P_r in the comparator l02, and the resultant signal is applied to the output regulator 8. This output regulator 8 calculates an excitation voltage V to be applied to the secondary winding lS of the generator motor l so as to develop the drive output demanded by the composite drive output command signal (P_o + ε). Especially, among functions for determining the quantities of excitation of the individual phases of the secondary winding lS of the generator motor l, thereby controlling the internal phase difference angle Δδ, a-phase, b-phase and c-phase voltages Va, Vb and Vc of the secondary winding lS of the generator motor l are given as follows:
where E is a voltage value determined by slip S and operating conditions of the variable-speed generator motor, δ_o is the internal phase difference angle determined by the operating conditions of the variable-speed generator motor, and Δδ is the internal phase difference angle controlled by the output signal of the output controller 8.
In the control of the variable-speed generator motor according to the equation (2), the voltage E is to be controlled according to the reactive power control command, and the internal phase difference angle Δδ is to be controlled according to the effective power control command. Thus, effective power is used as information for controlling the internal phase difference angle Δδ of the secondary winding lS of the generator motor l. That is, the internal phase difference angle Δδ is expressed as follows:

Δδ = ∫k₁(P_r - P_o - ε)dt + k₂(P_r - P_o - ε) ... (3)

where k₁, k₂ are constants. Calculation of the equation (3) in the output regulator 8 is so-called proportional plus integral calculation.
The signal representing the internal phase difference angle Δδ thus calculated is applied from the output controller l0 to the APPSC 3. In the APPSC 3, the firing of the thyristors of the frequency converter 4 is controlled to control the exciting voltage V applied to the secondary winding lS of the generator motor l, so that a rotating magnetic field corresponding to the slip frequency f₂ can be applied to the rotor, and the value of the exciting voltage V meets the actual internal phase difference angle Δδ. By the above manner of control, the drive output P_M (= P_r) of the generator motor l follows up the composite drive output command signal (P_o + ε) as shown in Fig. 7(d). This output control system constituted by the electrical circuits has a delay time determined substantially by, for example, the integrating function of the output regulator 8. This delay time is quite short as compared to a delay in the response of the rotation speed and the guide vane opening, and the drive output P_M follows up the signal (P_o + ε) without any appreciable delay.
On the other hand, the guide-vane opening command signal Y_OPT generated from the function generator l3 responding to the incremented output command signal P_o is increased as shown in Fig. 7(c). Y_r delays slightly relative to the change of P_o due to the effect of a delay inherent in the Y_OPT circuit or due to the presence of a delay element purposefully provided. Because of a speed limitation determined by the guide vane regulator 9 or due to a delayed operation of the hydraulic amplifier in the regulator 9, the response of the actual guide-vane opening Y_r is slow as seen in Fig. 7(c). Therefore, the mechanical input P_p changes at a rate slow relative to a change in the drive output P_M. Because of the above relation between the mechanical input P_p and the drive output P_M, the turbine/pump 6 is accelerated according to the difference between the drive output P_M and the mechanical input P_p as shown in the block 50 in Fig. 2, and the rotation speed N_r of the turbine/pump 6 increases. With the increase in the speed N_r, the error between N_r and N_OPT decreases until finally the relation N_OPT = N_r is attained. Also, because the relation Y_OPT = Y_r is satisfied by the guide vane regulator 9, the operation of the turbine/pump 6 is stabilized.
The reason why ε = O is achieved under a steady state, that is, how P_o = P_r is achieved in a steady state will now be described. As described above, the mechanical input P_p corresponds to the actual guide-vane opening Y_r. As also described above, the relation Y_OPT = Y_r holds.
Further, according to the operating principle of the optimum guide-vane opening function generator l3, the relation Y_OPT = P_o or equivalent holds.
On the other hand, the combination of the output regulator 8 (including the integrating element) and the P_r negtive feedback circuit provides the relation P_o + ε = P_r. Further, as described already, the relation P_r = P_M holds. Also, considering the integrating action of the inertia effect part l05 and the action of the N_r negative feedback circuit, the relation P_M = P_p holds.
From the above, it can be concluded that the relation P_o + ε = P_r = P_M = P_p= Y_r or equivalent = Y _OPT or equivalent = P_o holds, and the value of ε becomes finally zero.
Thus, in spite of the application of both the drive output control and the rotation speed control to the same motor l, both these controls can be reliably executed without any contradiction therebetween. In Fig. 7, the drive output control is principal, while the rotation speed control is a supplemental corrective control, and the response of the drive output P_M of the generator motor l can sufficiently follow up the command within several seconds. Thus, the variable-speed pumped-storage power generating system of the present invention can simply follow up the AFC command. Fig. 8 shows response characteristics when the output command signal P_o commands a decreased output, but its description is unnecessary since it can be sufficiently understood from Fig. 7.
The present invention has been described by reference to its application to a variable-speed pumped-storage power generating system of secondary excitation type as shown in Fig. l. The present invention is also applicable to a pumped-storage power generating system of a type as shown in Fig. 9 in which a synchronous generator motor lʹ driving a turbine/pump 6 is connected at its primary winding lP to an electric power system 5 through a frequency converter 4. Although, in this case, the frequency of the primary winding lP and the rotation speed of the turbine/pump 6 are proportional to each other, the rotation speed of the generator motor lʹ can be made variable by controlling the frequency of voltage supplied to the primary winding lP independently of the frequency of voltage in the electric power system 5. Also, the drive output of the generator motor lʹ can be made variable by regulating the quantity of power supplied to the primary winding lP.
The above description has referred to the operation after the pumping operation has been normally started. Consider now the stage where the guide vanes start to be opened prior to the pumping operation. This pumping starting stage includes depression of the water level, rotation of the pump/turbine under a no-loaded condition, release of the water-level depressing pressure to permit rise of the water level, opening the guide vanes, and starting the pumping operation. In this stage, the rate of increase in the pump input, after the guide valves are opened, is generally sharp immediately after the guide valves start to be opened. Unless this abrupt change in the pump input is appropriately dealt with, the rotation speed of the generator motor decreases greatly or deviates from the allowable band of the variable speed. On the other hand, when the frequency converter is designed so that the excitation control can deal with such an abrupt change in the pump input occurring in the stage of starting the pumping mode, the current and other ratings of the drive motor, frequency converter, etc. must be increased, resulting in an uneconomical design.
Fig. l0 shows a modification of the function generator l2 which overcomes such a problem. Referring to Fig. l0, a bias N₀ is applied through a contact lll, which is closed in the pumping operation starting stage (that is, before and immediately after the guide vanes are opened), to an adder ll0 to be added to an optimum rotation speed signal N_OPTʹ generated from a function generator l2-l. Thus, at the beginning of the pumping operation starting stage, the turbine/pump 6 is driven at the speed N_OPT = N_OPTʹ + N₀, and, even when the rotation speed decreases temporarily due to the opening of the guide vanes, the rotation speed of the pump can be maintained at a value close to the optimum speed N_OPTʹ. In this connection, continuous application of the bias N₀ to the adder ll0 is not desirable, and application of the bias N₀ should be suitably stopped when the operation seems to be stabilized after the guide vanes are opened.
The variable-speed generator motor l shown in Fig. l is of the type which has a primary winding and a secondary winding, which is connected at its primary winding to an electric power system, and whose secondary winding is a.c. excited by a frequency converter so that its rotor is rotated at a speed different from its synchronous speed. In the generator motor of such a type, a problem as described below arises when it is driven at a speed close to its synchronous speed. That is, the frequency converter exciting the secondary winding of the variable-speed generator motor is composed of anti-parallel connected thyristors. In this frequency converter, current is concentrated in specific thyristors for a long period of time when the generator motor is driven at the speed close to the synchronous speed, and, owing to the resultant rise of heat, the current capacity of the frequency converter greatly decreases.
The rotation speed range in which such a decrease in the current capacity of the frequency converter occurs, is called a forbidden band. It is apparent that the capacity of a frequency converter, for which the presence of the forbidden band is acknowledged and which is designed so as not to operate continuously in the forbidden band, is far smaller than that of a frequency converter for which the presence of the forbidden band is not acknowledged and which is designed so as to be capable of continuous operation in the forbidden band. Therefore, there has been an alternative between two designs. According to one of them, an economical frequency converter having the forbidden band is designed, and continuous drive output regulation in the forbidden band is abandoned. According to the other, no restriction is imposed on the operable region, and an expensive frequency converter having no forbidden band is designed.
Fig. llA shows a modification of the frequency converter l2 which deals with the prior art problem. Referring to Fig. llA, a nonlinear circuit l2-2 is provided besides a primary frequency generator l2-l determining an optimum rotation speed N_OPTʹ, and the optimum rotation speed signal N_OPTʹ generated from the nonlinear circuit l2-2 appears as an output signal N_OPT. The nonlinear characteristic of the nonlinear circuit l2-2 is determined by the synchronous speed N₁ of the generataor motor l and its margin α. The margin α is commonly in the order of 0.5 to l% of N₁. When the value of the margin α is as described above, N_OPT = N_OPTʹ when N_OPTʹ ≦ N₁ - α or when N_OPTʹ ≧ N₁ + α; N_OPT = N₁ - α when N₁ - α < N_OPTʹ < N₁; and N_OPT = N₁ + α when N₁ < N_OPTʹ < N₁ + α. Consequently, when the output command signal P_o increases proportionally as shown in Fig. llB, and the output N_OPTʹ of the function generator l2-l increases also in proportion to P_o, the output N_OPT of the non-linear circuit l2-2 is limited to (N₁ - α) or (N₁ + α) when N_OPTʹ is close to the synchronous speed N₁, although N_OPT increases in proportion to N_OPTʹ up to (N₁ - α) or after (N₁ + α). Thus, the synchronous speed N₁ can be quickly passed, and the undesirable temperature rise of the thyristors of the frequency converter can be suppressed to less than an allowable temperature range. Thus, according to this method, a variable-speed pumped-storage power generation system can be realized in which the frequency converter is of an economical design having a forbidden band and operable without any restriction in its operating condition.
It will be understood from the foregoing detailed description that the present invention provides a variable-speed pumped-storage power generating system which can greatly contribute to stable control including AFC control or such frequency control of an a.c. electric power system even in a pumping mode.

Claims

1. A variable-speed pumped-storage power generating system including a generator/motor (l) having a primary winding connected to an electric/power system (5) and a secondary winding connected to said electric power system through a frequency converter (4), and a turbine/pump (6) directly coupled to the shaft of said generator/motor, said power generating system comprising means (l0) for controlling the firing angle of said frequency converter according to an error signal between an output command signal commanding an output power required for said power generating system and a power signal indicative of an actual output power of said generator/motor.

2. A variable-speed pumped-storage power generating system according to Claim l, wherein said output command signal commanding the output power required for said power generating system is applied from a central load-dispatching office generally controlling said electric power system and includes an automatic frequency control signal of the electric power system.

3. A variable-speed pumped-storage power generating system including a generator/motor (l) having a primary winding connected to an electric power system (5) and a secondary winding connected to said electric power system through a frequency converter (4), and a turbine/pump (6) directly coupled to the shaft of said generator/motor, said power generating system comprising means (l0) for producing a target power signal by adding an error signal representing the difference between an actual rotation speed and a target rotation speed of said generator motor to an outpt command signal commanding an output power required for said power generating system, and controlling the firing angle of said frequency converter according to an error signal between said target power signal and a power feedback signal feeding back an actual output power of said generator/motor.

4. A variable-speed pumped-storage power generating system according to Claim 3, wherein said output command signal commanding the output power required for said power generating system is applied from a central load-dispatching office generally controlling said electric power system and includes an automatic frequency control signal of the electric power system.

5. A variable-speed pumped-storage power generating system including a generator/motor (l) having a primary winding connected to an electric power system (5) and a secondary winding connected to said electric power system through a frequency converter (4), and a turbine/pump (6) directly coupled to the shaft of said generator motor, said power generating system comprising means (l0) for controlling the firing angle of said frequency converter so as to provide a predetermined electrical output power and a predetermined rotation speed of said generator/motor in said power generating system.

6. A variable-speed pumped-storage power generating system including a generator/motor (l) having a primary winding connected to an electric power system (5) and a secondary winding connected to said electric power system through a frequency converter (4), and a tubine/pump (6) coupled directly to the shaft of said generator/motor and having guide vanes whose opening is variable, said power generating system comprising means (l0) for controlling the firing angle of said frequency generator so as to provide a predetermined electrical output power and a predetermined rotation speed of said generator/motor in said power generating system, and means (30) for controlling the opening of the guide vanes of said turbine/pump so as to provide an optimum guide-vane opening.

7. A variable-speed pumped-storage power generating system including a generator/motor (l) having a primary winding connected to an electric power system (5) and a secondary winding connected to said electric power system through a frequency converter (4), and turbine/pump (6) directly coupled to the shaft of said generator/motor and having guide vanes (7) whose opening is variable, said power generating system comprising a first function generator (l2) generating a target rotation speed signal on the basis of at least an output command signal commanding an output power required for said power generating system, a speed regulator (l6) generating an output signal corresponding to an error signal between the outpt signal of said first function generator and a rotation speed signal representing an actual rotation speed of said generator/motor, an adder (l0l) adding the output signal of said speed regulator to said output command signal commanding the output power required for said power generating system thereby generating a composite target output command signal, an output regulator (8) generating an output signal corresponding to an error signal between the output signal of said adder and a power signal representing an actual output power of said generator/motor, the firing angle of said frequency converter (4) being controlled by the output signal of said output controller, a second function generator (l3) generating a target guide-vane opening signal on the basis of at least said output command signal commanding the output power required for said power generating system, and a guide-vane opening regulator (9) generating an output signal corresponding to an error signal between the output signal of said second function generator and a guide-vane opening signal representing an actual opening of the guide vanes of said turbine/pump thereby controlling the opening of the guide vanes of said turbine/pump.

8. A variable-speed pumped-storage power generating system including a generator/motor (l) having a primary winding connected to an electric power system (5) and a secondary winding connected to said electric power system through a frequency converter (4), and a turbine/pump (6) directly coupled to the shaft of said generator/motor, said power generating system comprising a first function generator (l2) generating a target rotation speed signal on the basis of at least an output command signal commanding an output power required for said power generating system, a speed regulator (l6) generating an output signal corresponding to an error signal between the output signal of said first function generator and a rotation speed signal representing an actual rotation speed of said generator/motor, an adder (l0l) adding the output signal of said speed regulator to said output command signal commanding the output power required for said power generating system thereby generating a composite target output command signal, and an output regulator (8) generating an output signal corresponding to an error signal between the output signal of said adder and a power signal representing an actual output power of said generator/motor, the firing angle of said frequency converter (4) being controlled by the output signal of said output controller, said first function generator setting the rotation speed of said generator/motor at a relatively high speed when said power generating system starts its pumping operation.

9. A variable-speed pumped-storage power generating system including a generator/motor (l) having a primary winding connected to an electric power system (5) and a secondary winding connected to said electric power system through a frequency converter (4), and a turbine/pump (6) directly coupled to the shaft of said generator/motor, said power generating system comprising a first function generator (l2) generating a target rotation speed signal on the basis of at least an output command signal commanding an output power required for said power generating system, a speed regulator (l6) generating an output signal corresponding to an error signal between the output signal of said first function generator and a rotation speed signal representing an actual rotation speed of said generator/motor, an adder (l0l) adding the output signal of said speed regulator to said output command signal commanding the output power required for said power generating system thereby generating a composite target output command signal, and an output regulator (8) generating an output signal corresponding to an error signal between the output signal of said adder and a power signal representing an actual output power of said generator/motor, the firing angle of said frequency converter (4) being controlled by the output signal of said output controller, said first function generator providing a target rotation speed which changes abruptly in the vicinity of the rated rotation speed of said generator/motor so as to prevent said generator/motor from continuously operating at a speed close to the rated rotation speed.

l0. A variable-speed pumped-storage power generating system including a generator/motor (l) having a primary winding connected through a frequency converter (4) to an electric power system (5) and a secondary winding mounted on its rotary shaft, and a turbine/pump (6) directly coupled to the shaft of said generator/motor, said power generating system comprising means (l0) for controlling the firing angle of said frequency converter according to an error signal between an output command signal commanding an output power required for said power generating system and a power signal indicative of an actual output power of said generator/motor.

11. A variable-speed pumped-storage power generating system including a generator/motor (l) having a primary winding connected through a frequency converter (4) to an electric power system (5) and a secondary winding mounted on its rotary shaft, and a turbine/pump (6) directly coupled to the shaft of said generator/motor, said power generating system comprising means (l0) for producing a target power signal by adding an error signal representing the difference between an actual rotation speed and a target rotation speed of said generator/motor to an output command signal commanding an output power required for said power generating system, and controlling the firing angle of said frequency converter according to an error signal between said target power signal and a power feedback signal feeding back an actual output power of said generator/motor.

12. A variable-speed pumped-storage power generating system including a generator/motor (l) having a primary winding connected through a frequency converter (4) to an electric power system (5) and a secondary winding mounted on its rotary shaft, and a turbine/pump (6) directly coupled to the shaft of said generator/motor, said power generating system comprising a first function generator (l2) generating a target rotation speed signal on the basis of at least an output command signal commanding an output power required for said power generating system, a speed regulator (l6) generating an output signal corresponding to an error signal between the output signal of said first function generator and a rotation speed signal representing an actual rotation speed of said generator/motor, an adder (l0l) adding the output signal of said speed regulator to said output command signal commanding the output power required for said power generating system thereby generating a composite target output command signal, an output regulator (8) generating an output signal corresponding to an error signal between the output signal of said adder and a power signal representing an actual output power of said generator/motor, the firing angle of said frequency converter (4) being controlled by the output signal of said output controller, a second function generator (l3) generating a target guide-vane opening signal on the basis of at least said output command signal commanding the output power required for said power generating system, and a guide-vane opening regulator (9) generating an output signal corresponding to an error signal between the output signal of said second function generator and a guide-vane opening signal representing an actual opening of the guide vanes of said turbine/pump thereby controlling the opening of the guide vanes of said turbine/pump.